Tuesday, March 15, 2016

Sedimentation Patterns Bay Of Bengal: How Old Is The River Ganga

...In our legends it is said that the goddess Ganga's descent from the heavens would have split the earth had Lord Shiva not tamed here torrent by tying it into his ash-smeared locks. To hear this story is to see the river in a certain way: as a heavenly braid, for instance, an immense rope of water, unfurling through a wide and thirsty plain. That there is a further twist to the tale becomes apparent only in the final stages of the river's journey - and this part of the story always comes as a surprise, because it is never told and thus never imagined. It is this : there is a point at which the braid comes undone; where Lord Shiva's matted hair is washed apart into a vast knotted tangle. Once past that point the river throws off its bindings and separates into hundreds, maybe thousands of tangled strands....

Amitav Ghosh- The Hungry Tide

A study by K.S Krishna and colleagues published recently in Current Science shows very elegantly using sesimic reflection profiles and sediment isopach maps how the Bay of Bengal has been filling up with sediment since Late Cretaceous times.

The Bay of Bengal (BoB) originated with the rifting of India from Antarctica by early Cretaceous, thus forming the Indian east coast margin. The depression over time evolved into an ocean basin with new oceanic lithosphere forming in the Bay of Bengal at sea floor spreading centres. Its conjugate oceanic crust is probably beneath the Enderby Basin at the margin of East Antarctica. In this depression, an enormous volume of sediment has been deposited from mid-late Cretaceous to recent times. The age ranges of sediment packages, their distribution, geometry and thickness tell us about the changing source regions of these sediments and the influence of the rising Himalayas and the monsoons on sedimentation history.

Scientists involved in this study used seismic reflection profiles to construct a seismic stratigraphy of the sediment pile in the BoB. The ages ranges of the sesimic sequence were then calibrated using biostratigraphy erected from two deep sediment cores from the vicinity of the seismic lines. Thus, a Cretaceous to recent subdivision of the mega sequence into distinct depositional episodes separated by unconformities could be recognized. The seismic profiles also revealed the geometry and thicknesses of the sedimentary sequences and the topography of the basement.

Several coast perpendicular grabens (linear depressions) were clearly outlined. These are continuations of ancient Archaean and Proterozoic sutures zones and rifts now occupied by the major rivers of Peninsular India, the Cauvery, Krishna, Godavari and the Mahanadi. Sedimentary packages until the late Oligocene are thicker near the east coast and thin out into the deeper shelf areas. From early Miocene onwards the sediment packages are thicker near the Ganges Brahmaputra delta and in the deeper shelf areas to the east and north and thin towards the shallower coastal shelf. This implies a changes in direction of sediment delivery systems, with the Indian craton being the major source in the earlier phase, to the Himalayas and the Indo Burman ranges being the major source in the younger phase.

This is brought out beautifully by sediment isopach maps constructed for several time slices. Isopach maps show the thickness of sediments for a particular time slice. In this case four time slices were used: 1) Basement to Late Cretaceous 2) Late Cretaceous to Oligocene 3) Oligocene top to Late Miocene 4) Late Pleisocene to recent.

The results are shown below and they clearly show changing sediment sources and distribution pathways.

From Cretaceous to the Oligocene, thick sediment wedges coincide with the Peninsular river grabens indicating that sediments derived by erosion of the Indian craton was the major source to the BoB.

This indicates that river systems eroding the rising Himalayas were now the major suppliers of sediment to the BoB. Sediment thickness from late Miocene to mid Pleistocene also show enhanced sedimentation from the north and this pattern coincides with an increase in the Asian monsoon. From Pleistocene to recent times, there has been more sediment from the Godavari Krishna system to the Bob, while a strong sediment delivery system from the Ganges-Brahmaputra continues.

This is the end of part 1 of the post, but I had an intriguing question..

How old is the river Ganga of the plains, flowing from the Himalayan front near Haridwar to the Bay of Bengal? What does the data from the Bengal Basin tell us about paleo-rivers and how do geologists go about collecting and analyzing this data?

To answer this we go to the Bengal Basin, currently the site of the enormous Ganges-Brahmaputra delta, so eloquently described by Amitav Ghose in his book The Hungry Tide, and to the Gangetic plains and to the Cenozoic foreland basin sediments in Nepal and Uttarakhand in India.

Let's begin with this graphic which shows the evolution of the Himalayan orogen and the drainage systems as India collided with Asia in the Cenozoic

You can see that geologists think that by Miocene times (beginning ~ 24 million years ago), the Himalayan orogen had enough relief to have produced a distinct drainage system along its southerly flanks. Streams flowing transverse to the orogen met an axial river (paleo-Ganga) which took the sediments to the Bay of Bengal. The river Brahmaputra also is seen to have a Miocene history as well. The general agreement then is that the Ganga would have been flowing into the Bay of Bengal by at least 20 million years ago (Early Miocene) or so.

What is the evidence for this?

Another powerful use of isopach maps of the sedimentary sequence deposited in the Bengal Basin brings out Miocene drainage networks. The figure below of the tectonic elements of the Bengal Basin is from a paper by Ashraf Uddin and Neil Lundberg. It also shows the location of wells that provided the sediment thickness and sand/shale ratio data.

Sediment thickness and sand/shale plots of the Middle to Late Miocene Surma Group sediments (Lower Miocene plots show a similar pattern) clearly outline two major sediment lobes representing sand rich river channel networks flowing from the N and ENE source. These have been interpreted to be delta distributory channels of a major river. The sediment mineralogy of these Miocene sediments has also been studied. The sediments are made up of metamorphic and sedimentary lithic grains, plagioclase and K feldspar and a variety of minerals like pyroxenes, amphiboles, epidote and tourmaline, indicating exhumation and erosion of granites, granitic gniesses and a variety of metamorphic schists that make up the Himalayan orogen to the north and the Indo-Burman ranges to the east.

Sources and therefore paleo-drainage has also been pinpointed by 40Ar/39Ar dating of detrital white micas found in the Surma Group. 40Ar/39Ar is a thermochronometer. This is a radiometric dating method which gives the age of the closure temperature of the mineral i.e. the temperature below which there is no diffusion of the parent or daughter products into the external environment. The timing of major faulting events in the Himalayas have been calculated using this method. For example, in the Bhuban Formation of the Surma Group sediments, the 40Ar/39Ar dates of white mica show a strong 16 million year peak. This corresponds to the timing of movement of the Main Central Thrust in the Sikkim Himalayas. Movement along the Main Central Thrust generated enough heat to cause the growth of white micas in the vicinity of the fault zone. 40Ar is derived from the decay of radiogenic 40K in white mica. So, a 16 million year old 40Ar/39Ar date for white micas in the Bengal Basin sediments suggest erosion of rocks uplifted by the Main Central Thrust in the Sikkim Himalaya!

All these data taken together suggest the presence of a paleo-Brahmaputra flowing southwards from the eastern Himalaya syntaxis with additional drainage from the eroding eastern Indo Burman ranges. The sediment route brought out by the isopach maps and sand percentage maps show that this channel system in the Miocene flowed through the upper Assam valley and eastwards of the Shillong plateau and entered the Bengal Basin via the Sylhet depression. Sediments were then transported further south deeper into the Bay of Bengal. It is only in more recent times (Pleistocene) that the uplift of the Shillong plateau has forced the Brahmaputra to change course. It now takes a westerly turn and wraps itself along the northern margin of the Shillong plateau before turning south into the Bengal Basin. Fig Source: Uddin A. and Lundberg N. 1999

What about the paleo-Ganga?

The data from well Hazipur 1 is quite revealing. It is far away from the influence of the paleo-Brahmaputra of the Miocene (don't get confused by its proximity to the present location of the Brahmaputra). It also shows a high percentage of sand suggesting a major river channel west of the paleo-Brahmaputra. In the western Bengal Basin, seismic stratigraphy has also shown the presence of Miocene age mega channels flowing south. These channels indicate the presence of another river distributary system and strongly hint that a Miocene paleo-Ganga was reaching the Bengal Basin then. Unfortunately, no sediment mineralogy work is available that could pinpoint the source of these westerly Miocene channel sediments.

The search for the paleo-Ganga takes us northwestwards into the Himalayan foreland and alluvial plains of Nepal and Uttarakhand, India. Is there any physical sedimentological evidence of large Miocene river channels flowing southeastwards roughly parallel to the Himalayn orogen, as the Ganga does today?

Geologists have extensively studied the Cenozoic foreland basin that developed in response to the loading and flexure of the lithosphere due to the Himalayan orogeny. Essentially a big linear moat or a trough formed in front of the rising Himalayas. This moat contains sediments including the Siwalik Group, which was deposited by rivers draining the Himalayas in the Miocene. Below is a reconstruction of the depositional environment of the Siwalik Group.

It shows that major river channels flowed perpendicular to the Himalayan thrust front and deposited sediments in alluvial floodplains and in meandering and braided stream channels. Large alluvial fans were formed due to this style of deposition. The situation is analogous to the present day river system in which south flowing transverse rivers like the Ramganga, Goriganga, Kosi and the Gandaki draining the Uttarakhand and Nepal Himalaya join an axial Ganga river flowing southeastwards. Paleocurrent analysis of Siwalik sediments show a southerly current direction. This implies a transverse drainage network like the present day Ramganga and Kosi. There is so far no evidence for a Miocene axial river (paleo-Ganga) flowing parallel to the Himalayan orogen in the Siwalik Group of sediments.

That doesn't mean that there was no Miocene Ganga, but that the exposed Siwaliks are the wrong place to look for it. An axial river would have developed a little further south of the alluvial fan complexes prograding from the orogen front and so older axial river channels are likely buried beneath the Gangetic Pleistocene-Holocene alluvium. The oldest sediment deposited by the Ganga which is currently exposed in the Gangetic plains is about 120 thousand years old. But there is 2-3 km of older sediments ranging in age from Miocene to mid Pleistocene underneath this recent alluvium. In effect, the Siwaliks continue in the subsurface of the Gangetic plains. This next map shows the tectonic elements of the Himalayan system and sediment thicknesses in the Gangetic plains.

It shows that the exposed Siwaliks ranges (in map- southern boundary of frontal folded zone) are just the deformed northernmost part of the foreland basin . All those transverse streams eroding the Himalayas and depositing sediment in the proximal part of the Siwaliks would have met an axial river flowing in the distal parts of the foreland, more to the south of the orogenic front.

There is another geochemical clue that a paleo-Ganga and paleo-Brahmaputra along with a paleo-Indus were draining the Himalayas since the Miocene and that evidence lies in the changing 87Sr (Strontium) isotope and 187Os (Osmium) isotope composition of sea water. The reasoning is that the uplift of the Himalayas exposed enormous volumes of new rock which were rich in these elements. Chemical weathering of these rocks leached these elements into rivers and ultimately into the ocean. Take a look at the plot of changing 87Sr and 187Os isotope of sea water through the Cenozoic. The Sr isotope composition of ancient sea water is estimated by measuring Sr incorporated in CaCO3 shells of marine creatures. The Os isotope composition of sea water is estimated by measuring Os incorporated in manganese hydroxide nodules precipitated in deep sea sediments.

In the plot you can clearly see an enrichment of 87Sr isotope in the early part of the Miocene. This has been interpreted to signal the exhumation and erosion of radiogenic 87Sr rich granitic rocks of the Greater Himalayan Crystalline Series uplifted by the movement along the Main Central Thrust. At around 16 million years ago, there is a flattening in the 87Sr signal indicating decreased erosion of the Greater Himalayas. This is paralleled at 16 million years ago by an increase in the 187Os content of sea water. This change has been interpreted to signal the uplift, exhumation and weathering of the Lesser Himalayas.

Neoproterozoic to Cambrian strata of the Lesser Himalayan Krol Tal Group contain organic and phosphatic black shales which are enriched in radiogenic 187Os. This Krol-Tal belt is exposed all across the strike of the Himalayan orogen from Pakistan to the west through Himachal Pradesh and Uttarkhand in India, to Bhutan in the east. So, the changing sea water chemistry through the Cenozoic strongly hints at the erosion of these rocks by the Gangetic system rivers along with the Indus and Brahmaputra systems beginning by around 16 million years ago.

I can argue like a lawyer and point out that since physical evidence of a paleo-Ganga as an axial river is missing, the changing sea water chemistry could be due to combined contributions of the Indus and Brahmaputra. Or for that matter, erosion anywhere in the world of Sr and Os rich rocks in the Miocene could explain the change in composition of the well mixed global ocean So, is there any evidence that Ganga system tributaries were enriched in 187Os during the Miocene and that they were contributing this element to the ocean?

Here, geologists have found out that beginning around 15 million years ago, Mn (manganese) oxide nodules precipitated in the floodplains of river deposits are enriched in 187Os. The weathering and erosion of Krol-Tal strata released clay and silt size particles which got deposited in floodplains. These floodplain sediments were chemically weathered and released 187Os, which due to its affinity to Mn got incorporated in the growing Mn oxide nodules in the soils forming on floodplain sediments. The ground water that was carrying this 187Os also released that element in the river. These transverse rivers would have joined an axial river, the paleo-Ganga to the south, thus carrying the 187Os signal to the Bay of Bengal.

All this sound circumstantial, but those transverse rivers would have eventually had to join a larger axial river just as they do today. Where else could they have terminated?

As you might have guessed all this has been more of an intellectual exercise for me over the past few weeks. Most of the research studies don't directly address the question of a paleo-Ganga. Rather, the emphasis of Himalayan studies is on understanding the sequential uplift of lithotectonic terrains and their erosional history via its signal as preserved in the Cenozoic foreland basin and Bay of Bengal sediments. I've had to pick my way through several research studies to tease out the bits relevant to this question. A long email exchange with Ashraf Uddin, my friend from Florida State University days who has worked extensively on Bengal Basin sediments also helped.

Looking at the entirety of the data I will say that the evidence for a Miocene paleo Ganga is not as clear or direct as it is for a paleo Brahmaputra, but that is because of a lack of detailed studies. Provenance work on those Miocene mega channels in west Bangladesh and seismic profiling of the Gangetic plains should surely reveal a Miocene Ganga. Presently the Ganga breaches the Siwalik hills at Haridwar. In the past before the Siwaliks existed, beginning around 20 million years ago, there would have been a large mountain stream originating from the great heights of the Greater Himalaya, flowing through the expanse of the Lesser Himalaya and onto the plains somewhere near present day Haridwar, and then funneled by the elongate foreland trough, taking a southeasterly course towards the Bay of Bengal.

10 comments:

Nice Suvrat. Perhaps coincidental perhaps not that the Godavari is called the 'Vriddha Ganga'. The Ganga is that which comes down as rain and deposits sediment in the Bay of Bengal[does that have an older name?]

In ancient Hindu scriptures, this water body is referred to as 'Mahodadhi' (Sanskrit: महोदधि, lit. great water receptacle)[5][6] while it appears as Sinus Gangeticus or Gangeticus Sinus, meaning "Gulf of the Ganges", in ancient maps.[7]

I read the whole account, trying to figure out the location of paleao-Ganga before Himalayan orogeny, since it is known to be antecedent. Did it exist as axial river at the northern periphery of peninsula with Chambal, Betwa, Ken etc flowing into it. Or did the closure of Tethys result in formation of Ganga. I am a biologist and hence a layman for geology. A simple answer is desired. Prakash Nautiyal

thanks for reading- The Indus and Brahmaputra are antecedent. They originate on the Asian plate in Tibet and so would have existed before the India Asia collision and the rise of the Himalaya. The Ganga is not antecedent. It originates in the Greater Himalayan ranges which is uplifted Indian crust. So its origin begins after the closure of Tethys and establishment of continental conditions. A southerly drainage (paleo-Yamuna and paleo Ganga) would have developed as the Himalayas started rising.

Before the Tethys closed, peninsular Indian rivers flowing north northeastwards similar to the Chambal and Betwa would have flowed into the Tethys sea and only later after the closure of the Tethys in the early Miocene would have been captured by an axial Yamuna-Ganga drainage system.

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ABOUT THIS BLOG

I am a Sedimentary Geologist. On Rapid Uplift I write mostly about topics within the geosciences, but sometimes on biological evolution and environmental issues. I like to travel and in my free time I teach 12 year old kids soccer and rugby.